Hi Peter
Have you considered doing a cost comparison between Black Cabs against some of the Scottish tour companies that run organised tours in 15 seat minibuses from Edinburgh thoughout the Highlands and back.
The good thing is that they run the routes for most of the year round.
They appear to get closer to the tourists and tourism hotspots throughout Scotland which may spread your message to a wider niche market. You know the types..... cash rich who like adventure, no problem with travelling to get a unique experience etc.
You will probably have considered this but it's worth a thought.
Although I appreciate that you may possibly get a more visual impact on the black cabs and they hang around (commuter routes) airports, railway and bus stations.
It may be worth looking at the high visibility ads on the Black Cabs and pick out your top 5 and then contact the companies who are advertising and simply ask them how the numbers stack up (ROI)
John